The invention relates to compensation for wavelength changes in laser interferometer measurement systems, and in particular to a wavelength tracking compensator with an ultra-low thermal expansion optical cavity.
Laser interferometers measure linear displacements to sub-micron accuracies for measurement and control of precision equipment, such as integrated circuit manufacturing, disc-drive manufacturing and precision measurement and machining equipment.
Laser interferometers use the wavelength of the laser light as a standard for making distance measurements. Any changes in the measurement environment that affect the apparent wavelength of the laser light can seriously degrade measurement accuracy. Thus, changes in atmospheric conditions such as pressure, temperature, composition and humidity, which change the optical index of the medium through which the laser beam passes, can introduce errors into the measurement.
Many precision positioning and measuring machines have a built in temperature control, but no control for pressure or other atmospheric factors. The atmospheric factors can be eliminated by providing a vacuum environment for the laser system, but this is very expensive and is usually impractical in manufacturing applications.
It is possible to directly measure the individual atmospheric factors, but this is also expensive, requiring multiple sensors. This method also has inherent response delay caused by the time constants of the sensors, and requires calculation of the cumulative effects of the changes on the laser light wavelength.
One attempt to correct for wavelength changes in precision machine tool applications was a compensator comprised of a steel cavity mounted on a plane mirror interferometer optic. The thermal expansion of the cavity approximately matched the thermal expansion of the machine tool on which it was mounted. However, with a steel cavity the wavelength compensation is only accurate for use on steel equipment with approximately the same thermal expansion coefficient; furthermore the unit must be used in a temperature controlled environment to insure that the steel cavity is at the same temperature as the machine tool it is monitoring. These problems limit the performance of the instrument to a relative standard and limit the application of this type of instrument primarily to the machine tool industry.
The wavelength tracking compensator of the invention is a device for tracking changes in the wavelength of the laser light source of an interferometer system by monitoring changes in the apparent length of a stable physical standard measured by the system. This standard is an optical cavity formed by two plane mirrors optically contacted to the ends of a rigid, stable tube that is open to the surrounding air. A double path, differential interferometer is used to monitor the apparent changes in the cavity's length caused by actual changes in the index of refraction of the surrounding air.
For the wavelength tracking compensator, it would be ideal to use an optical cavity constructed of material having a very low thermal expansion coefficient. The problem arises in mounting the cavity on the machine or instrument to be monitored, which will have a quite different thermal expansion coefficient.
The optical cavity must be supported by a structure which applies a constant supporting force to the cavity regardless of changes in the ambient temperature and pressure. Any change in the external force due to changes in ambient temperature or pressure would change the optical length of the cavity or distort the optical figure of the mirrored surfaces at either end of the cavity. The support structure must maintain a radial clearance of very small variation to prevent cavity tilt which would result in cosine error. The support structure must prevent roll or translation of the cavity with respect to the interferometer to maintain optical alignment between the cavity and the laser beam.
Conventional support structures could be adjusted to work at one temperature. As the temperature deviated from the "set point" temperature, the cavity would either become loose in its mounts causing excessive cosine error or tight in its mounts causing a length change in the cavity and/or distortion of the mirror's optical figure.
There are several difficulties in mounting and aligning the interferometer to achieve optimal performance. The support structure must provide three rotational degrees of freedom (roll, pitch and yaw).
The double path, differential interferometer must be very accurately aligned with the optical cavity to prevent any cosine error in the cavity's optical path length measurement, to prevent polarization leakage, and to maximize optical power at the receiver. The mount for the interferometer must provide three rotational degrees of freedom (roll, pitch and yaw) to achieve proper alignment. The mount must provide roll adjustment to eliminate optical leakage of the vertical polarization of light from the laser head into the horizontal polarization and vice versa. The mount must provide pitch and yaw adjustments to allow the incident beams to be adjusted so they are exactly perpendicular to the cavity's mirrored surfaces to prevent cosine error and to maximize optical power at the receiver.
Previous interferometer mounts provided discrete pitch and yaw adjustments with no roll capability at all. These mounts typically had two machined parts (a pitch plate and a yaw plate) and required four screws and four washers to mount and adjust the assembly. The pitch and yaw motions would have to be adjusted one at a time and often the act of clamping disturbed the aligned position, forcing the user to realign the interferometer repeatedly until satisfactory alignment was achieved. The roll angle of the mount, not being adjustable, was at the mercy of the machining tolerances of the individual components providing the pitch and yaw adjustments. Accumulated tolerance build-up could result in a considerable amount of roll axis error, degrading system performance.
To measure the wavelength change, the differential interferometer uses two sets of beams, one reflected by the front surface of the stable cavity and the other reflected by the rear surface of the cavity. These two beam components are mixed and the difference in their frequencies is used as the measure frequency. The measure frequency is compared to a reference frequency generated by the laser head. Any phase change between the two frequencies is due to wavelength change of the laser beam through the cavity.
Conventional laser measurement systems measure the cumulative phase difference between the reference and measure frequencies, starting at some arbitrary point in time, typically when the system is initialized. If one of the beams is blocked, or the signal for one of the beams is lost, even momentarily, the measurement must be re-initialized because the phase information during the interruption cannot be reconstructed. This causes difficulties in making measurements that remain accurate over long time periods.
An object of the invention is to provide a system for accurately measuring the changes in the wavelength of the laser light and compensating the laser interferometer measurement for those changes. The system eliminates the compensation inherent in other methods which involve multiple sensors.
Another object of the invention is to provide a temperature compensated mount for an optical cavity made of an ultra-low thermal expansion material, so the wavelength tracking compensator can be used with great accuracy in a non-temperature controlled environment. This greatly increases the range of applications and the performance of the compensator while reducing the requirements on controlling the environment.
Another object of the invention is to provide a semi-kinematic mount for optical components capable of simultaneous adjustment in three degrees of rotational freedom (roll, pitch and yaw), and securely held in proper alignment during clamping.
Another object of the invention is to provide a phase measurement system for an interferometer that can recover from measurement interruptions without re-initialization and that has extended phase measurement resolution.
The preferred embodiment of the invention comprises a stable physical standard optical cavity and a double path, differential interferometer mounted on a baseplate. The interferometer monitors the apparent changes in the cavity's length caused by actual changes in the index of refraction of the surrounding air. The cavity is supported with a temperature compensated mount that utilizes the principle of differential thermal expansion of two different metals to create a mount which exactly matches the coefficient of expansion of the cavity material. A semi-kinematic mount supports the double path, differential interferometer and allows the interferometer to be accurately aligned to the cavity with three rotational degrees of freedom to prevent any cosine error in the cavity's optical path length measurement, to prevent polarization leakage, and to maximize optical power at the receiver. A phase measurement system interpolates between interference fringes to extend measurement resolution and provides absolute phase information relative to the fringes so that the measurement system can prevent measurement uncertainty caused by a beam interruption and recover from an interruption without reinitialization.